This invention relates to passive fiber optic multiplexers, and specifically to a wavelength responsive multiplexer useful for combining a plurality of signals of different light wavelengths carried by different optical fibers onto a single optical fiber, or for separating signals of different light wavelengths carried by a common optical fiber for continued transmission on a pair of optical fibers.
Fiber optic multiplexing, in the past, has been accomplished using active systems in which a pair of waveguides are surrounded by a crystal material which has a voltage responsive refractive index. Such active systems require the application of power, as well as careful tuning and adjustment during use, for proper functioning.
In addition to these active systems, passive frequency selective couplers have been devised, such as that described in U.S. Pat. No. 3,957,341, issued May 18, 1976, to Henry F. Taylor, in which the waveguides have different phase propagation constants at all frequencies except for the frequency to be coupled, yielding a separation of this frequency from all other frequencies. Such systems, however, require careful material selection for the desired frequency separation and do not lend themselves to adjustment during or after manufacture to accomplish the separation of optical signals over a broad range of frequencies.
While the frequency dependence of coupling light energy between two identical parallel dielectric waveguides has been theoretically predicted in a number of published scientific articles, such coupling of light energy in passive systems has generally been considered impractical for most applications, since the frequency selectivity has generally been considered relatively unpronounced, that is, such systems have generally been considered to yield only relatively poor resolution.
There exists, therefore, a need for a passive optical coupler which provides high resolution frequency selection and which is adjustable to provide such resolution for selected frequencies within a broad frequency band. Thus, in many optical systems, it is desirable that multiple signals be simultaneously transmitted on a single optical fiber at different optical frequencies, thus multiplying the transmission capability of the fiber. Multiplexers for use in such systems are preferably tunable, at least at the time of their manufacture, to permit the multiplexing or separation of different frequencies so that, at a particular point of use, a single one of the transmitted frequencies may be detected. It is desirable that such multiplexing be accomplished with the lowest possible throughput losses so that system efficiency will not be unduly limited by losses in the multiplexer.